The last 50 years of research on language development have revealed in young language learners two, potential conflicting, abilities. On the one hand, infants are exquisitely sensitive to specific surface properties of their input and the statistics thereof. On the other hand, infants are able to rapidly generalize beyond their input based on highly abstract formal properties. The relative importance of sensitivity to surface vs. abstract linguistic properties is at the heart of theoretical debates about the nature of human language. The research proposed here reflects our ongoing attempt to determine how language learners combine their sensitivity to surface properties of language and their ability to abstract beyond these properties. From the research performed during the current period of funding, we have formed the following 3 hypotheses: (1) Memory demands, task demands and previous experience all influence the level of abstraction achieved for formally identical input. (2) Language learners form only the most abstract representation necessary to account for the set of data they have encountered. (3) Under some circumstances, adults with subtle developmental language disabilities are less likely than adults with normal language to generalize based on abstract properties of language. To test these hypotheses, participants are familiarized with stimuli from an artificial language-like system. They are then tested on new stimuli that are either consistent or inconsistent with the particular properties of the familiarization stimuli under study. Significant discrimination of consistent vs. inconsistent test items is taken as evidence of learning. Using this paradigm, the research compares learning across different input conditions (2 formally identical language systems with different surface instantiations, formal systems that conform vs. do not conform to properties of natural languages) and across populations (6- to -12-month-old infants, adults with normal language, adults with impaired language abilities). The findings from this research are likely to reveal important information about how infants and adults are influenced by the specifics of their language input as they generalize beyond it. The findings also have the potential to provide new insights into the role of learning in language disorders.